How to create rust procedurally?
Hope the image is self explanatory enough. Hope it gives you a good starting point.

Some Explaination

Click on the image to view it in full resolution. You can copy the nodes and will get the same results. The Musgrave Texture are used as the mask to create the rust patch. A Second Musgrave Texture is given variation in the values, acts as the mask for the silver non rusty parts of the material. The noise material just provide a little color variation for the silver material, so it is not flat and boring.

The node pretty much tells you how it's constructed. The bottom image shows the closeup of the material, just for a visual reference that in closeup the material is still pretty convincing.





Additional Side Notes


  Regarding the UV image, I use it for visual testing uv unwrap to
  prevent tile stretching on none procedure map, for this case I did not
  use it at all as it is indeed a procedural material, so no uv work is
  required on the model at all. It's just the default setup for my
  workflow when I start a new file.